doc: mention trigger helper functions in CREATE TRIGGER docs

Reported-by: petermpallesen@gmail.com

Discussion: https://postgr.es/m/159195294959.673.5752624528747900508@wrigleys.postgresql.org

Backpatch-through: 9.5

Author: bmomjian

doc/src/sgml/ref/create_trigger.sgml

@@ -457,6 +457,12 @@ UPDATE OF <replaceable>column_name1</replaceable> [, <replaceable>column_name2</
    value did not change.
   </para>
 
+  <para>
+   There are a few built-in trigger functions that can be used to
+   solve common problems without having to write your own trigger code;
+   see <xref linkend="functions-trigger"/>.
+  </para>
+
   <para>
    In a <literal>BEFORE</literal> trigger, the <literal>WHEN</literal> condition is
    evaluated just before the function is or would be executed, so using